In disk drives, defects on the media surface can cause the read channel to repeatedly detect incorrect data (hard errors). Very large defects may result in hard errors that are too long for the disk drive ECC algorithm to correct. Defect scans are used in the manufacturing process to flag those sectors with large defects so they are excluded from use during normal drive operation.
At the factory, based upon the number of detected defects, it may be determined whether the disk drive is useable or not. The disk drive may fail the manufacturing process when too many defects are detected. Conventionally, if the disk drive is determined to be usable given the number and severity of the detected defects, a predetermined space around the detected defect is designated as a margin, which becomes designated as being thereafter unavailable for user data. Moreover, although every effort is made to seal the drive during manufacturing, contaminants may still become sealed therein. Such contaminants may, after the drive is shipped to the customer and put into use, cause scratches on the media. Some scratches, or portions thereof, are sufficiently shallow as to be properly handled by the error correction in the firmware. Other scratches may be too deep for the firmware error correction to correct. Conventionally, media defects are detected, mapped and margined during the manufacturing process. Consequently, when a drive leaves the factory, is shipped to the customer and put to use in the field, it is assumed that the probability of finding any new defects is low. Therefore, the drive's capacity to accommodate new defects in the field, such as scratches, is limited. Such media defects that occur or are discovered as a result of user operations of the disk drive, however, do occur and should be addressed.